1. Field of the Invention
The present invention relates to a magnet switch for starter for starting an engine.
2. Description of Related Art
There have been proposed starters in which a starter motor is operated in two steps to bring a pinion into an engagement with a ring gear of an engine smoothly. In such starters as disclosed for example in JP-U No. 59-30564, a voltage lower than a rated voltage for the motor is applied through a fixed resistor connected to a motor circuit until the pinion is brought into engagement with the ring gear, and short-circuits the fixed resistor after the pinion and the ring gear have been engaged to apply the rated voltage to the motor.
When the motor is thus actuated in two steps, the motor operates at a low rotating speed until the pinion and the ring gear are engaged and, consequently, impulsive force or shock that acts on the pinion and the ring gear when the pinion and the ring gear are engaged can be reduced to prevent damaging the tooth surfaces of the pinion and the ring gear.
When the impulsive force that acts on the pinion and the ring gear when the same are engaged is reduced, the strength of a speed reduction mechanism of a reduction type starter, which is prevalently used to improve starting performance or to reduce the size, may be reduced.
Reduction in torque for actuating the motor reduces a tangential force for rotating the pinion. Consequently, the pinion can be satisfactorily advanced toward and brought into engagement with the ring gear by a reduced thrust. Therefore, the thrust can be produced by a magnet switch that generates a reduced attraction and the magnet switch may be of a relatively small size.
When the starter is used in a cold district, frost is formed on the surface of a stationary contact built in the magnet switch when the starter held in a high-temperature, high-humidity atmosphere during the operation of the engine is cooled rapidly after the engine has been stopped and the frost may possibly be frozen to ice the surface of the contact.
Since an electric wiring cable of a large diameter connected to a battery is connected to the power terminal of the magnet switch, the cable starts cooling down first after the engine has been stopped, the power terminal of the magnet switch connected to the cable cools down accordingly, and then the stationary contact formed integrally with the power terminal cools down.
If highly humid air prevails in the magnet switch, moisture contained in the air condenses in drops of dew, the drops of dew change into frost and finally ice.
When a compact-sized magnetic switch capable producing a relatively low attraction is used under such condition, impulsive force that is exerted on the stationary contact of the magnet switch by the movable contact when the movable contact is brought into contact with the stationary contact is relatively low. Therefore, if the surface of the stationary contact is iced, the movable contact and the stationary contact can not be satisfactorily connected, which may increase the probability of malfunction.
For preventing malfunction due to the icing of the contact, a starter disclosed in JP-U No. 54-88563 employs a magnet switch having a stationary contact and a movable contact at least one of which has a surface provided with a plurality of grooves to enable layers of ice formed on the surfaces of the contacts to be easily broken upon the collision of the movable contact and the stationary contact so that the movable contact is able to come into satisfactory electrical contact with the stationary contact.
The starter of this type, however, is unable to give full play to its effect when the compact-sized magnet switch is used because such compact magnet switch exerts only a reduced impulsive force on the contacts when the movable contact is brought into contact with the stationary contact.
Other starters disclosed in JP-U Nos. 50-9635 and 51-32342 employ a heating wire disposed so as to surround contacts of a magnet switch to melt ice formed on the surfaces of the contacts by heat generated by the heating wire.
The starters with such heating wire that emits radiant heat to melt the ice takes a relatively much time. Therefore, it is unable to meet the user's requirement that the engine be started as soon as possible, and needs parts for controlling power supply to the heating wire increasing the cost.